Fuel cells are projected for use as a power source for electric vehicles and other applications. A fuel cell is a device which converts the energy of a chemical reaction into electricity. It differs from a battery in that the fuel and oxygen are stored external to the cell, which can generate power as long as the fuel and oxygen are supplied. A fuel cell having a solid polymer electrolyte is known as a polymer-electrolyte-membrane fuel cell (PEM FC).
A PEM FC typically comprises a first electrode, (i.e., an anode), a second electrode, (i.e., a cathode) and a solid polymer electrolyte membrane. The cathode and the anode are secured to opposite sides of the membrane so that the cathode is separated from the anode by the solid polymer electrolyte membrane. The fuel, typically hydrogen, is supplied to the anode, and oxygen, typically in the form of air, is supplied to the cathode. The hydrogen molecules disassociate at the anode to form protons and electrons and the oxygen molecules dissociate at the cathode to form oxygen atoms. The protons pass through the polymer electrolyte membrane from the anode to the cathode to react with the dissociated oxygen atoms formed at the cathode while the electrons, which produce the electric current, traverse an external circuit. The protons, electrons and oxygen recombine at the cathode to form water.
Each electrode (i.e., the anode and the cathode) comprises a catalyst to facilitate the reactions occurring thereat. The catalyst is typically a particulate noble metal such as platinum and is dispersed and supported on a high surface area support material.
The support material in the PEM fuel cell typically consists exclusively of carbon particles. Carbon has good electrical conductivity which helps facilitate the passage of the protons and electrons from the catalyst and electrode to the polymer electrolyte membrane and to the external circuit. To promote the formation and transfer of the protons and electrons, and to prevent drying out of the membrane, the fuel cells are operated under hygroscopic conditions. To generate these aqueous condition, the solid polymer electrolyte membrane is usually hydrated (i.e., by boiling in water) prior to its introduction into the fuel cell and the hydrogen fuel and oxygen gases are humidified prior to entry into the fuel cell.
Notwithstanding the good electrical conductivity of carbon, carbon is relatively hydrophobic, and as such, the boundary contact between the reactive gases, the water and the surface of the solid electrodes made exclusively of carbon contributes to high electrical contact resistance and ohmic power loss in the fuel cell. This diminishes the efficiency of the fuel cell. Accordingly, it is an object of the present invention to provide an electrode for use in a polymer-electrolyte-membrane fuel cell which has lower resistance and less ohmic power losses than electrodes which employ the use of carbon material exclusively as its support material.
Moreover, the majority of the costs associated with electrodes is attributed to the high cost of the noble metal which makes up the catalyst. Only those catalytic sites exposed on the surface of the catalytic particles contribute to the catalytic activity of the electrode, and thus, electrodes with the highest fraction of the noble metals accessible to the reaction, i.e., those with the highest dispersion, are optimal. The extent of dispersion of the noble metal catalyst on the support material, and the stability of such high dispersion in use, i.e., resistance of the catalyst against agglomeration, is directly related to the surface area and the availability of surface sites on which the dispersed noble metal can be anchored. Carbon support material typically has a surface area of about 10-50 m.sup.2 /g and a relatively low surface density of available anchoring surface sites. This is because carbon materials are for the most part graphitized. In graphitic carbons, the major part of the exposed surface consists of chemically inert basal planes with the "edge" planes on which the anchoring sites are located representing only a small percentage of the carbon support surface.
It would be desirable to provide a catalytic support which has a higher stable surface area and also a higher surface density of anchoring surface sites than catalytic supports consisting exclusively of carbon. This would increase the dispersion of the noble metal catalyst and thus limit the amount of catalyst needed. As such, it is another object of the present invention to provide a fuel cell polymer-electrolyte-membrane electrode which can be made less costly than electrodes having exclusively carbon support material.
Furthermore, it is desirable to provide an unimpeded access of the gaseous hydrogen fuel and oxygen reactants to the active noble metal catalysts in order to avoid diffusional limitations on the amount of power drawn from the electrode. Accordingly, it is another object of the present invention to provide a catalyst support which has improved gas permeability relative to supports made exclusively of carbon.
Disclosure of the Invention The present invention meets the above and other objects by providing an electrode for use in a polymer-electrolyte-membrane fuel cell. The electrode comprises catalyst support material comprising conductive particulate zeolite material, and noble metal catalysts supported on the catalyst support material.
Moreover, the present invention further provides a polymer-electrolyte-membrane fuel cell comprising an ionomeric, conducting polymer membrane, a first electrode on a first face of the polymer membrane, and a second electrode on a second face of the polymer membrane. At least one of the electrodes comprises a catalyst support material comprising conductive zeolite particulate material, and noble metal catalysts supported on the catalyst support material.
The conductive zeolite material contains acidic protonic entities on its surface which make it more hydrophilic than carbon and, thus, when used as a catalyst support in PEM FC electrodes, results in lower resistance and less ohmic power losses than electrodes which employ the use of carbon material exclusively as its support material. The conductive zeolite material also enables a relatively high dispersion of the catalytic noble metals and, because of its array of channels, allows for a relatively high gas permeability.